The term vapour retarder is somewhat undefined The broadest

The term “vapour retarder” is somewhat undefined. The broadest definition could be that a vapour retarder is less vapour tight than the minimum recommendation for vapour barriers and more vapour tight than the maximum recommendation for wind barriers. According to Norwegian recommendations this would give the following range in vapour resistance for vapour retarders: 0.5m3.4m. Vapour retarders that are said to allow drying to the interior are now sold on the European market. These vapour retarders vary quite a lot in regard to their level of vapour resistance, typically Sd=2–10m. While a vapour retarder has a given constant vapour resistance, there are some vapour barriers sold on the European and North American market with adaptable vapour resistance in regard to what is actually needed. A popular term for these products are “smart” or “intelligent” vapour barriers. The physical behaviour of these products varies, but the main principle is that the vapour barrier should function as an ordinary vapour tight vapour barrier most of the time, preventing vapour Annexin V-FITC Apoptosis Kit into the construction from the indoor air. If, on the other hand, the construction is wet, for example due to built-in-moisture or leakages, so that the relative humidity (RH) on the exterior side of the vapour retarder gets high, the vapour resistance will be reduced so that there may be possibilities for drying inwards. One such product is for example claimed to have an Sd-value at approximately 4–5m when the RH is below about 40% and Sd=0.1–0.2 when the RH is above 80% (Künzel, 1996). It is important to note that in this analysis we have only considered what the maximum vapour resistance of the vapour retarder should be, given that a relatively large proportion of the total drying should be directed inwards. We have not considered here whether the relatively low vapour resistance of the vapour retarder (Sd below 10m, and possibly even below 2m) has negative consequences for condensation and risk of mould growth, due to outward vapour transport from indoor air. The subject of the minimum acceptable vapour resistance on the warm side to avoid moisture problems has however been investigated in numerous other publications, and a review is given in Vinha (2007).
Method
Results and discussion
Conclusions Modern wood frame structures in Nordic countries have usually relatively vapour open wind barriers, often with an Sd-value around and below 0.1m. The simulations show clearly that the inward directed drying is modest in such constructions, since most of the moisture will dry outwards. To get an inward drying of some significance (25% or more of the total drying) the vapour resistance on the warm side must be pretty low, typically with an Sd-value lower than 1–2m. If the wind barrier is very vapour open (for example Sd<0.02m) an even smaller percentage of the total drying will be inward directed. In order to maintain about 25% inward directed drying, the vapour resistance on the warm side must be reduced further, for example lower than Sd=0.75m. On the contrary, a more vapour tight wind barrier (for example Sd=0.5m) will only require a vapour resistance on the warm side lower than approximately Sd=3m in order to maintain minimum 25% inward directed drying. However, the study shows clearly that it is more effective to reduce the vapour resistance on the cold side than on the warm side, in order to improve the overall drying of built-in-moisture, accidental leaks and the like. For example, it is much more efficient to reduce the vapour resistance of the wind barrier from Sd=0.1m to 0.01m (or from Sd=0.5m to 0.1m), than to reduce the vapour resistance on the warm side from Sd=10m to 2m.